April 26, 2020

#LeetCode: Longest Common Subsequence

Given two strings text1 and text2, return the length of their longest common subsequence.

A subsequence of a string is a new string generated from the original string with some characters(can be none) deleted without changing the relative order of the remaining characters. (eg, "ace" is a subsequence of "abcde" while "aec" is not). A common subsequence of two strings is a subsequence that is common to both strings.

If there is no common subsequence, return 0.

Example 1:
Input: text1 = "abcde", text2 = "ace"
Output: 3
Explanation: The longest common subsequence is "ace" and its length is 3.

Example 2:
Input: text1 = "abc", text2 = "abc"
Output: 3
Explanation: The longest common subsequence is "abc" and its length is 3.

Example 3:
Input: text1 = "abc", text2 = "def"
Output: 0
Explanation: There is no such common subsequence, so the result is 0.

Constraints:
1 <= text1.length <= 1000
1 <= text2.length <= 1000
The input strings consist of lowercase English characters only.

Let us understand this problem, by taking a couple of examples:

In each example, we start comparing each character in s2 with each character of s3. If they are equal, we are joining then with a line. If any of the line intersect the other one, we need to ignore one of these lines while finding the longest common sub-sequence.

Algorithm 1: Finding the longest common sub-sequence using recursion.

Let's understand this algorithm by taking an example:

GIT URL: Java Solution of Leet Code's Longest Common Subsequence problem by using Recursion
Java Solution 1:

This solution is rejected by Leet because of 'Time Limit Exceeded' for inputs "pmjghexybyrgzczy" and "hafcdqbgncrcbihkd".

Algorithm 2: Finding the longest common sub-sequence using memorization.

Algorithm 1 is taking exponential time. If you check the example where we are passing 'bd' and 'abcd', you will notice we are calling the recursive function twice for values 'd' and 'c' (which is again calling the recursive function to get the longest common subsequence).

Instead of calling the recursive function again and again for the same parameters, we can make use of memorization. In this technique we store the results for each recursive call's and while executing the recursive function we will check if the result is already computed. If yes, we will return the result, else calculate it.

Steps:
1). Create a memorization table lcs (two dimensional array) of text1.length+1 and text2.length+1.
2). Since we have created 2 dimensional array of int type, value at any index is zero.
3). We will start comparing each character of text1 with each character of text2, and while doing so we will fill our memorization table.
4). If text1[i-1]=text2[j-1], we will set lcs[i][j]=1 + value at left diagonally upward position of memorization table (lcs[i-1][j-1]).
5). Else set lcs[i][j]=value at exactly upper position or value at exactly left position, which ever is higher.
6). Longest Common Subsequence would be the value at the last index of memorization table (i.e lcs [m][n]).

GIT URL: Java Solution of Leet Code's Longest Common Sub-sequence problem by using memorization table
Java Solution 2:

ALSO CHECK: Other LeetCode Solutions in Java

-K Himaanshu Shuklaa..

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